The present invention relates generally to a torque detection apparatus, and more particularly to such a torque detection apparatus arranged so as to measure the magnitude of a torque on the basis of a relative rotational displacement of a driven shaft with respect to a drive shaft. The present invention may be employed, for example, in an electric power steering system for electrically power-assisting a steering device of a motor vehicle by means of an electric motor.
Hydraulic power steering systems, generally known as power steering systems, are operated by using hydraulic pressure. There are problems which arise with this type of power steering system, however, in that the mounting position thereof on a vehicle is limited because of provision of hydraulic-pressure pipes and further in that there is the possibility of oil leakage from the hydraulic pressure pipes. A further problem arising in such a hydraulic power steering system relates to an increase in the amount of fuel consumption because a hydraulic pressure pump is always driven irrespective of steering operation. Recently, taking into account the above-mentioned problems, electric power steering systems have been developed to electrically effect the power-assistance of the steering by the aid of an electric motor, the electric motor being driven by means of a control circuit only in response to operation of a steering wheel of the vehicle, thus resulting in free selection of the mounting position and improvement of the fuel consumption. This electric power steering system further allows easy and appropriate control of the production torque in correspondence with variation of the vehicle speed by control of the supply current to the electric motor. On the other hand, an important problem in such an electric power steering system relates to the need for provision of an apparatus which can accurately detect a torque produced with respect to a steering shaft. One known technique is a stain-gauge type torque detection apparatus in which the torque detection is made with a sensor being brought into direct contact with a rotational shaft of the vehicle. This type of the detection apparatus, however, causes reduction of stability and reliability on the detection because of direct contact with the rotational shaft. Another known technique involves detection of variation of a magnetic permeability with stress variation occurring due to the load torque. Apparatus of this type results in that difficulty is encountered when attempting to accurately measure the torque because of a small sensor output and influence to the output characteristic due to temperature. A further known system is of the phase-difference type, as disclosed in Japanese Patent Provisional Publication No. 61-223525, in which a rotational displacement between two rotating bodies due to a torsion is detected with a torsion bar being interposed therebetween. However, in this system, the measurement of the torque is allowed only under the condition that the rotating bodies are in the rotating states and therefore difficulty is encountered to measure a static torque. Japanese Utility Model Provisional Publication No. 55-36354 discloses a still further type of torque detection apparatus. However, this type of apparatus is arranged to measure a torsion amount on the basis of the absolute value of a magnetic flux density which is varied in accordance with torsion variation, and therefore the output to be obtained is also small.
Thus, a further improvement would be required from the viewpoint of improvement of reliability, size-reduction, accuracy of measurement of a static torque, and so on.